1. Field of the Invention
The present invention relates to semiconductor electronic devices, and, more particularly, to transversal filters.
2. Description of the Related Art
Aircraft operational requirements for communications, navigation, and identification equipment have been increasing continuously and have resulted in a proliferation of complex and expensive equipment to do each specific function. The diversity of signal characteristics applied in communications, navigation, and identification presents a formidable array of challenges in the design of signal processors. Therefore, the challenge is to develop high performance, efficient, programmable front end processors that are applicable in a wide variety of systems. In particular, programmability of the filter function is the basis for integrating several communications functions into a single device; without programmability, integration would consist only of making everything smaller, not of combining functions, and flexibility for adapting to changes in requirements and waveforms would be lost.
Transversal filters tap (detect) an input signal at various places along the signal's path and output a weighted sum of the tap signals; thus a transversal filter is ideally a process of shift, multiply and sum. Transversal filters find use in various communication functions; and if the weights multiplying the tap signals can be programmed, then such filters could be used, for example, in programmable communications receivers and adaptive signal processing for spread-spectrum systems. Indeed, the programmable transversal filter (PTF) is an extremely versatile wideband signal processor. This single device can operate as a bandpass, band-reject, adaptable or matched filter.
Surface acoustic wave (SAW) devices are well suited for carrying the input signal in a PTF because the acoustic signal can be sensed at locations along the propagation path by a series of taps that minimally degrade the acoustic signal. The SAW device provides a tapped delay line for the input signal. The tap weights can be electronically programmed by use of dual-gate MOS field effect transistors for the tapping and weighting functions: one gate is connected to the tap and the other gate is connected to the tap weight control, the outputs are tied together for the summing function. See, S. Kwan et al, Dual-Gate Depletion-Mode DMOS Transistor for Linear Gain-Control Application, 26 IEEE Tran. Elec. Dev. 1053 (1979), which describes a monolithic programmable transversal filter (PTF) fabricated on a silicon substrate with a rectangular ZnO thin film acting as the SAW device, the input signals are converted to acoustic signals by a wide-band interdigitated input transducer on the ZnO, the taps are a series of electrodes across the ZnO with each electrode end connected to a first gate of a dual-gate depletion-mode common-source configured silicon MOSFET adjacent the ZnO, the second gates of the MOSFETs are connected to external tap weight controls, and the drains of all of the MOSFETs on one side of the ZnO are tied together to the inverting input of a differential output amplifier in the silicon and the drains of all of the MOSFETs on the other side of the ZnO are tied to the noninverting input of the differential amplifier. This provides a differencing scheme and linear dependence of the MOSFET gain on the tap weight control voltage.
Several other PTFs using SAW devices have been reported in the literature. Recently a SAW/FET (LiNbO.sub.3 SAW with silicon MOSFETs and air-gap coupled) approach demonstrated 50 MHz of bandwidth centered at 150 MHz. However, tap control range was limited to 16 dB and single tap insertion loss was 80 dB; see, D. Oates et al, Wide-Band SAW/FET Programmable Transversal Filter, Proc. IEEE Ultrasonics Symp. 312 (1984). A monolithic GaAs approach in which the SAW and the FETs are implemented on the same GaAs substrate has demonstrated 58 dB dynamic range at 500 MHz over a 50 MHz bandwidth (10%); see, J. Duquesnoy et al, A Monolithic 7 Tap-Programmable Transversal Filter on Gallium Arsenide, Proc. IEEE Ultrasonics Symp. 303 (1984). As noted by the authors, "no good programmable device has reached the industrial level so far, despite all the work done in that field for the last 15 years."
A variety of other approaches to programmable filters have been developed, but fail to overcome the problems of the known programmable transversal filters such as limited dynamic range (100 dB in a 1 MHz bandwidth seems necessary for general applications). These other approaches include: (1) A GaAS CCD delay line with fixed tap weights, the filter's center frequency is programmed by the clock frequency; see, I. Deyhimy et al, GaAs and Related Heterojunction Charge-Coupled Devices, 27 IEEE Tran. Elec. Dev. 1172 (1980) and W. Hill et al, 1 GHZ Sample Rate GaAs CCD Transversal Filter, Proc. 1985 GaAS IC Symposium 27 (1985); (2) A cascade of GaAs sample/holds as a tapped delay line and an array of fixed capacitors for tap weighting, several capacitor arrays are included on a single chip to switch between lowpass, highpass, and bandpass responses; see, A. McKnight et al, High Frequency GaAs Transversal Filter, 20 Elec. Lett. 84 (1984); (3) A separate SAW delay line for each bit of tap weight programming accuracy; see, J. Lattanza et al, Programmable RF Signal Processors Demonstrated for Spread-Spectrum Communications Systems, Microwave System News 76-92 (April 1985); (4) A GaAs combined FET and acoustic device which takes advantage of the piezoelectric modulation of MESFET depletion regions by an acoustic wave; see, S. Merritt et al, GaAs SAW/MESFET Programmable Tapped Delay Line, Proc. 1984 IEEE Ultrasonics Symp. 308; and (5) An acoustic charge transport device; see, M. Hoskins et al, Buried Channel Acoustic Charge Transport Devices in GaAs, Proc. 1983 GaAs IC Symp. 96.
Poor tap weight control range and poor dynamic range have severely impaired performance of all PTFs reported to date. Tap weight control range limits filter sidelobe performance. Low dynamic ranges nullifies all the advantages of even the best sidelobe performance.